Glasses or thermoplastics (hereinafter "thermoplastics") are often in an unstable condition due to stresses acquired during processing; particularly, cooling stages. Such thermoplastics are said to be "strained", and if the strain is excessive, the thermoplastics may crack or break into pieces when handled or utilized in their conventional manner.
Annealing is generally defined as the process for removing or diminishing the strains and stresses in thermoplastics. It is often achieved by heating the substance to be annealed and then allowing it to gradually cool. Two successful techniques often employed when annealing thermoplastics include one which withdraws the thermoplastics from a heat source, and the other which causes the heat from the heat source to diminish. Both techniques are often referred to as bulk annealing techniques since they involve heating the interior and exterior of the thermoplastics. The former is generally achieved in lehrs and the latter is generally achieved in ovens. Additional techniques for bulk annealing thermoplastics include those which employ infrared radiation. While the above-mentioned techniques assist in reducing stains in thermoplastics, they are not always desirable since bulk techniques typically result in shrinkage of the thermoplastics as well as the deterioration of the physical and chemical properties of the thermoplastics.
It is of increasing interest to reduce the stresses and strains of thermoplastics without employing bulk annealing techniques such as the above-described. Accordingly, the instant invention is directed to a process for reducing strains in thermoplastics by surface annealing which unexpectedly and simultaneously preserves the physical and chemical properties of the thermoplastics.